Silicon carbide electrical heating elements are known to be susceptible to oxidation which substantially changes their electrical properties. U.S. Pat. No. 3,875,477 (Fredrikkson I) discloses an igniter whose porous recrystallized silicon carbide has an internal network of open porosity. This conventional igniter is produced by forming an igniter-shaped green body comprising fine and coarse SiC particles and firing this green body at about 2400.degree. C. in nitrogen. During the firing step, the highly reactive fine SiC particles vaporize and then redeposit on the coarse SiC particles, thereby forming a layer of "recrystallized" SiC which both coats and connects the coarse SiC particles. An example of this conventional recrystallized structure is shown in FIG. 1.
During use, the oxidizing atmosphere surrounding this igniter penetrates the igniter's porosity, the silicon carbide at the surface of these pores reacts with the oxygen to form silica, an electrical insulator, thereby decreasing the conductive cross-section of the SiC igniter, resulting in decreased amperage and an increased resistance (at a fixed voltage). This phenomenon is known as "aging". It has been found that the conventional recrystallized SiC igniter ages to such an extent that its resistivity increases over 6-12% after only 6000 hours of cycling (5 minutes on, 5 minutes off) at a service temperature of 1480.degree. C.
One proposal for minimizing the effects of oxidative aging in electrically conductive refractory bodies is described in U.S. Pat. No. 4,187,344 (Fredrikkson II). Porous SiC heating elements are coated with silicon nitride and/or silicon oxynitride particles in a liquid slurry and these particles are carried into the pores of the element to a depth of at least 6.4 mm. After the slurry has dried, the article is fired at about 1000.degree. C. to fix the silicon oxynitride or silicon nitride particles in place and form a barrier against undue oxidation. However, it was found in practice that water vapor and combustion products detrimentally react with the submicron sized silicon nitride/silicon oxynitride impregnant particles. In addition, it was found that the actual penetration produced by this method was less than 0.5 mm.
In other approaches, refractory silicon carbide elements as described in U.S. Pat. No. 3,492,153 are protected by reacting aluminum vapor with nitrogen gas within the pores of the silicon carbide article to form in-situ aluminum nitride. However, this structure does not have an acceptable life in a gas oven environment because of the reactivity between the water vapor in the gas flame and the aluminum nitride impregnant.
Attempts to make igniters more resistant to oxidative deterioration are also shown in U.S. Pat. Nos. 3,509,072; 3,875,476; 4,120,829; and 4,204,863. As each of these disclosures describes the use of various bonding compositions to improve serviceability, the electrical characteristics of the resulting igniters are materially altered.
The solution to peripheral oxidation proposed by U.S. Pat. No. 4,429,003 (Fredriksson III) comprises coating the porous SiC igniter with a slurry of fine silicon carbide particles. The slurry (which is applied by spraying, painting or vacuum impregnation), reportedly flows substantially through the entire porous phase of the body. The treated article is then subjected to an oxidizing atmosphere to convert the silicon carbide particles to silica. Since the silica molecules occupy more space than the SiC particles that were oxidized, their in-situ formation can seal off the pores from further diffusion of oxygen. However, it was found that the resulting silica layer tended to devitrify and undergo a phase change in use, and so was prone to flaking, thereby exposing the underlying igniter to an oxidizing environment. Other methods of providing a protective silica layer have also produced the problematic flaking.
In addition, it has been found that the methods of coating favored by the prior art (e.g., brushing or vacuum infiltration) fail to fully infiltrate the protective particles fully into the porosity of the conventional SiC igniter.
Sealing the porosity of the conventional SiC igniter with an external layer of CVD SiC has been proposed. However, not only is this method expensive, the resulting coated igniters have been found to display inconsistent aging behavior.
Therefore, there is a need for a porous silicon carbide igniter which is more resistant to aging. In particular, there is a need for a material which will remain within 4% of its original design resistivity over 6000 hours of cycling (5 minutes on, 5 minutes off) at a service temperature of 1480.degree. C.